Vehicle electronic stability control system including improved wheel speed detection

ABSTRACT

A vehicle control system includes a powertrain electronic control unit and an electronic stability control unit that is connected to the powertrain electronic control unit via a vehicle communication bus. The vehicle powertrain includes a wheel, a wheel speed sensor that is configured to detect a speed of the wheel, an electric motor that is configured to drive the wheel, and an electric motor speed sensor that is configured to detect the speed of the electric motor. If the vehicle is not traveling at low speeds (for example, less than 2 kilometers per hour), the electronic stability control unit controls the vehicle based on an output of the wheel speed sensor. However, if the vehicle is traveling at low speeds, the electronic stability control unit controls the vehicle based on an output of the electric motor speed sensor.

BACKGROUND

In electric vehicles, as in traditional vehicles, maintaining controlover the angular speed of the wheels is critical to maintaining tractionwith the road surface. A loss of friction can occur from excessiveacceleration or deceleration (i.e. hard braking). When accelerating, thewheels can receive excessive torque from the electric motor. This maycause a loss of traction with the road surface and wheel spin.Similarly, when the wheels receive excessive braking force, the wheelsmay lose traction with the road surface. Modern vehicle systems striveto eliminate a loss of traction and wheel spin with vehicle electronicstability control systems. The vehicle electronic stability controlsystems may include, for example, antilock braking, traction control,and/or stability control functions.

Electronic stability control systems are limited in effectiveness by theability of the vehicle's sensors to determine the vehicle's behavior. Insome situations, the vehicle's sensors provide inaccurate or misleadinginformation about the vehicle's behavior. This situation can arise evenwhen the sensors are performing their function correctly and accurately.For example, a wheel speed information is used by the electronicstability control system to perform antilock braking, traction control,and stability control functions. However, in some circumstances, forexample if the wheel is not maintaining traction with the road surface,the wheel speed sensor may be correctly detecting the wheel's rotationalspeed, but the information may inaccurately describe the vehicle'sspeed. Inaccurate information about the vehicle can cause the wheelstability control systems to underperform. In another example, acharacteristic of the wheel speed sensor is low resolution at lowvehicle speeds, for example for vehicle speeds between 0 kilometers perhour and 2 kilometers per hour. Such low resolution can negativelyaffect how the wheel stability control systems are implemented.Therefore it is desirable to have accurate wheel speed in all operatingcircumstances and at all operating speeds.

SUMMARY

For vehicles in which wheels are driven by an electric motor, forexample in electric or hybrid vehicles, the signals for electric motorspeed can be used by the electronic stability control system to replacethe wheel speed sensor signal in certain operating conditions, such aslow vehicle speeds. The signals for electric motor speed are alreadyavailable to use for most electric vehicles via the vehiclecommunication bus.

By using the signals for electric motor speed to replace the wheel speedsensor signal when the wheel is rotating slowly, several advantages andbenefits can be realized. In one example, in traction control systemapplications, use of the electric motor speed sensor permits measurementof the transition between wheel “rolling” (non-slip) and wheel“spinning,” (slip) for situations where the vehicle is accelerating froma standstill condition. The knowledge of this transition allows forimprovements to the friction (mu) estimation and traction control systeminitialization.

In another example, when operating the vehicle using driver-assistedparking functions and maneuvers, use of the electric motor speed sensorpermits both distance and driving direction calculations to besignificantly improved relative to the same calculations when made usingthe wheel speed sensor. For example, the relatively greater resolutionprovided by the electric motor speed sensor at low travel speedsprovides a more precise indication of distance traveled, which iscritical during a parking operation.

In another example, when operating the vehicle using Off-Road electronicstability control functions, cruise control off-road (CCO) and hilldescent control (HDC) functions, which operate at very low speeds, canbe improved by the added resolution obtained when using the electricmotor speed sensor.

In yet another example, software that detects whether a vehicle is or isnot at a standstill can be improved by the added resolution obtainedwhen using the electric motor speed sensor, allowing a more reliabledetermination of the standstill conditions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is schematic diagram illustrating a vehicle control system forpowertrain of an electric vehicle including an electric motor connectedto each of the right and at the left drive wheels, each of the left andright drive wheels being driven by the corresponding electric motor.

FIG. 2 illustrates an electric motor speed sensor mounted within anelectric motor so as to detect rotational speed and direction of a rotorof the electric motor.

FIG. 3 illustrates an output signal of the electric motor speed sensorof FIG. 2.

FIG. 4 illustrates a wheel speed sensor mounted on a wheel hub of adrive wheel.

FIG. 5 illustrates an output signal of the wheel speed sensor of FIG. 4.

FIG. 6 is schematic diagram illustrating an alternative embodimentpowertrain controlled by a vehicle control system of an electric vehicleincluding a single electric motor that is connected to and drives boththe right and left drive wheels.

FIG. 7 is a graph of vehicle speed versus time illustrating the wheelspeed, the electric motor speed and vehicle speed for a rear left wheelhaving a dedicated electric motor.

FIG. 8 is a graph of vehicle speed versus time illustrating the wheelspeed, the electric motor speed and vehicle speed for a rear right wheelhaving a dedicated electric motor.

FIG. 9 is a graph of vehicle speed versus time illustrating the wheelspeed for front left and right wheels having a common electric motor andshared axle, the electric motor speed and vehicle speed.

FIG. 10 is a flow chart illustrating a method of controlling the vehiclepowertrain via the vehicle control system.

DETAILED DESCRIPTION

FIG. 1 illustrates a vehicle control system 10 for a powertrain 11 of anelectric vehicle. In the powertrain 11, drive wheels, for example therear wheels 20, 21, are used to drive the vehicle and each of the rightrear wheel 20 and the left rear wheel 21 is driven by an electric motor28. A battery 16 is electrically coupled to a drive inverter 18, whichis electrically coupled to each electric motor 28. A powertrainelectronic control unit 12 (ECU) is connected to the battery 16, thedrive inverter 18, and the electric motors 28. The electric motors 28each include a motor shaft 34 that is mechanically coupled to a driveshaft 38 via a gear set 36. The drive shaft 38 is mechanically coupledto the corresponding drive wheel 20, 21. However, in other embodiments,the gear set 36 may be omitted and motor shaft 34 is also the driveshaft 38.

Referring to FIGS. 2 and 3, the electric motors 28 can be any type ofelectrical motor that has sufficient horsepower to drive the vehicle,including, for example, an alternating current (AC) motor or anelectrically commutated (EC) motor. Each electric motor 28 is controlledby the powertrain electronic control unit 12 via a vehicle communicationbus. The vehicle communication bus is an internal communications networkthat provides communication between the electronic components within thevehicle. The communication bus employed by the vehicle control system 10may be any appropriate communication bus such as a Controller AreaNetwork (CAN), Local Interconnect Network (LIN), FlexRay, etc.

Each electric motor 28 includes an electric motor speed sensor 30 thatis disposed within a housing 44 of the electric motor 28 and isconfigured to detect a speed of the electric motor 28. In someembodiments, the motor speed sensor 30 may be a rotor position sensorthat detects a positon of the motor rotor 40 with respect to the motorstator 42, and the speed of the electric motor 28 is calculated as afunction of change in rotor position and time. In the illustratedembodiment, the electric motor speed sensor 30 is a Hall effect sensorthat is mounted within the motor housing 44 adjacent to the motor rotor40. When the rotor magnetic poles pass the electric motor speed sensor30, a high (for one pole) or low (for the opposite pole) signal isgenerated. As seen in FIG. 3, an output signal from the electric motorspeed sensor 30 is a sinusoidal signal that alternates between positiveand negative maximums. The electric motor speed sensor 30 iselectrically connected to the powertrain electronic control unit 12 viathe vehicle communication bus.

The electric motor speed sensor 30 accurately detects the rotationalposition of the motor rotor 40 relative to the motor stator 42. Inaddition, the electric motor speed sensor 30 accurately detects thedirection of rotation of the motor rotor 40 relative to the motor stator42.

Referring again to FIG. 1, during acceleration, the battery 16 providespower to the drive inverter 18 by supplying the drive inverter 18 withdirect current (DC). The drive inverter 18 converts DC into alternatingcurrent (AC) for use in the electric motors 28. The electric motors 28convert electric power into mechanical power by driving the motor shaft34. The powertrain ECU 12 communicates with the drive inverter 18 andthe battery 16 to manage the power delivery to and from the electricmotors 28.

The vehicle control system 10 further includes a vehicle electronicstability control unit 14. The vehicle electronic stability control unit14 is configured to provide, for example, one or more of antilockbraking, traction control, stability control and/or other vehiclecontrol functions. The stability control unit 14 automatically controlsthe brakes (not shown) of individual wheels to control the heading ofthe vehicle for example preventing the heading from changing too quickly(spinning out) or not quickly enough (plowing out). To this end, thevehicle electronic stability control unit 14 receives as direct inputssignals from the wheel speed sensors 22, a brake pressure sensor (notshown) and a brake light switch (not shown). As used herein, the term“direct input” refers to a condition in which a sensor is directlyelectrically connected via a line that is not included in the vehiclecommunication bus, or is directly placed on the circuit board of thestability control unit 14. The vehicle electronic stability control unit14 receives other signals from the vehicle communication bus, typicallyincluding for example engine signals related to engine speed, enginetorque, throttle position, etc., a steering angle sensor, and signalsrelated to inertia measurement including outputs from a yaw rate sensor,a longitudinal acceleration sensor, a lateral acceleration sensor, etc.

Referring to FIGS. 4 and 5, in the illustrated embodiment, the wheelspeed sensors 22 each include a metal reluctor wheel or tone ring 25 anda magnetic detector 27. The tone ring 25 is annular and includesoutwardly protruding, equidistantly spaced teeth 26. Although theillustrated tone ring 25 includes 48 teeth, the tone ring 25 may includea greater or fewer number of teeth 26. The tone ring 25 surrounds, androtates concurrently with, the drive shaft 38 and wheel 20, 21. Thedetector 27 is fixed to the vehicle and thus does not rotate with thedrive shaft 38. In addition, the detector 27 is disposed facing the tonering 25 with a small gap therebetween (about 1 mm) so as to detectindividual metal teeth 26 as the wheel rotates. As seen in FIG. 5, anoutput signal from the detector 27 is formed of discrete pulses that arespaced apart in time corresponding to the passage of individual teethpast the detector 27.

Although the wheel speed sensors 22 provide reliable and accuratedetection of wheel speeds at normal vehicle traveling speeds, the wheelspeed sensors 22 have poor resolution at very low vehicle speeds, forexample at speed that are less than 1 to 2 kilometers per hour. Inaddition, the wheel speed sensor 22 can also be unreliable in detectingthe direction of rotation of the tone ring 25. For example, the detector27 must see multiple teeth to ensure that it is not detecting the sametooth multiple times and make a determination between a wheel that isrocking back and forth and a wheel that is moving in a single direction.

The vehicle electronic stability control unit 14 receives output fromthe wheel speed sensors 22 of each wheel 20, 21 and compares the wheelspeeds of each wheel 20, 21 to determine if wheel slip is occurring.Prior to comparing the wheel speeds of each wheel 20, 21, the electronicstability control unit 14 determines whether the electric motor speedsensor 30 is providing an output signal corresponding to a wheel speedthat is greater than a predetermined wheel speed. The predeterminedwheel speed generally corresponds to a wheel rotational speed abovewhich the wheel speed sensor 22 performs well for example a wheel speedcorresponding to normal vehicle travel speeds. Based on experimentaldata, the predetermined wheel speed for some vehicle configurations isabout 2 kilometers per hour, but the predetermined wheel speed may begreater or lesser depending on the specific application. If the electricmotor speed sensor 30 provides an output signal corresponding to a wheelspeed that is greater than the predetermined wheel speed, the vehicle isconsidered to be traveling at relatively high speeds and the electronicstability control unit 14 controls the electronic stability controldevices, including one or more of antilock braking, traction control,stability control and/or other vehicle control functions, based on anoutput of the wheel speed sensor 22, along with various other sensors inaccordance with known operation of these systems and devices.

However, if the electric motor speed sensor 30 provides an output signalcorresponding to a wheel speed that is less than or equal to thepredetermined wheel speed, the vehicle is considered to be traveling atlow speeds, and the electronic stability control unit 14 controls theelectronic stability control devices, including one or more of antilockbraking, traction control, stability control and/or other vehiclecontrol functions, based on an output of the electric motor speed sensor30, along with various other sensors in accordance with known operationof these systems and devices. Thus, at low vehicle speeds, theelectronic stability control unit 14 replaces the information obtainedfrom the wheel speed sensor 22 with information obtained from theelectric motor speed sensor 30, which is known to provide more accurateand timely wheel speed information at low vehicle speeds than the wheelspeed sensor 22.

Although the vehicle control system 10 has been described herein withrespect to the powertrain 11 in which each of the drive wheels 20, 21 isdriven by an electric motor 28, the vehicle control system is notlimited to being used with this powertrain 11. For example, in someembodiments, the vehicle control system 10 is used with an alternativeembodiment powertrain 110 in which both of the drive wheels 20, 21 aredriven by single, common electric motor 28 (see FIG. 6). In thealternative embodiment powertrain 110, common elements are referred tousing common reference numbers. As in the previously describedembodiment, during non-low speed operation of the vehicle, theelectronic stability control unit 14 controls the electronic stabilitycontrol devices based on an output of the wheel speed sensor 22, alongwith various other sensors in accordance with known operation of thesesystems and devices. However, during low speed operation of the vehicle,the electronic stability control unit 14 controls the electronicstability control devices based on an output of the electric motor speedsensor 30, along with various other sensors in accordance with knownoperation of these systems and devices.

FIG. 7 is a graph of vehicle speed versus time illustrating the wheelspeed, the electric motor speed and vehicle speed for a rear left wheelhaving a dedicated electric motor. FIG. 8 is a graph of vehicle speedversus time illustrating the wheel speed, the electric motor speed andvehicle speed for a rear right wheel having a dedicated electric motor.FIG. 9 is a graph of vehicle speed versus time illustrating the wheelspeed for front left and right wheels having a common electric motor andshared axle, the electric motor speed and vehicle speed. These graphsillustrate that for normal driving speeds (e.g., greater than 5kilometers per hour), the output of the wheel speed sensor 22 closelycorresponds to the output of the electric motor speed sensor 30. Inaddition, these graphs illustrate that there are slight differencesbetween the output of the wheel speed sensor 22 and the output of theelectric motor speed sensor 30 at low speeds (e.g., less than 2kilometers per hour. However, the output of the electric motor speedsensor 30 is more accurate and reliable at low speeds, and wheelrotation can be accurately measured by the electric motor speed sensor30 much earlier than by the wheel speed sensor 22, whereby using theelectric motor speed sensor 30 at low speeds provides more timely andaccurate control of the vehicle by the control systems.

Referring to FIG. 10, a method of controlling the vehicle powertrain 11via the vehicle control system 10 to use the electric motor speed sensor30 as a replacement sensor during low speed vehicle operation includesthe following method steps: Initially, the electronic stability controlunit 14 determines whether the electric motor speed sensor 30 isproviding an output signal corresponding to a wheel speed that isgreater than a predetermined wheel speed, for example in a rangecorresponding to a normal operating range of the wheel speed sensor 22(step 200). This determination may occur prior to a determination ofwheel slip of any of the wheels. If the vehicle electronic stabilitycontrol unit 14 determines that the electric motor speed sensor 30 isproviding an output signal corresponding to a wheel speed that isgreater than the predetermined wheel speed, the vehicle is considered tobe traveling at normal traveling speeds. In this case, the electronicstability control unit 14 controls the electronic stability controldevices, including one or more of antilock braking, traction control,stability control and/or other vehicle control functions, based on anoutput of the wheel speed sensor 22 and other sensors in accordance withknown operation of these systems and devices (step 202).

If the vehicle electronic stability control unit 14 determines that theelectric motor speed sensor 30 is not providing an output signalcorresponding to a wheel speed that is greater than the predeterminedwheel speed, the vehicle is considered to be traveling at low speeds. Inthis case, the electronic stability control unit 14 controls theelectronic stability control devices, including one or more of antilockbraking, traction control, stability control and/or other vehiclecontrol functions, based on an output of the electric motor speed sensor30 and other sensors in accordance with known operation of these systemsand devices (step 204).

It is contemplated that the electric motor speed sensor 30 could replacethe wheel speed sensor 22 as an input to the control system for allvehicle speeds. Doing so would require some signal processing(filtering, smoothing, etc.) for implementation, and would also reducevehicle costs due to a reduction in the number of sensors. However,there are advantages to using the wheel speed sensor 22 at normal travelspeeds and the electric motor speed sensor 30 at low travel speeds. Thisis due at least in part to the fact that the electric motor wheel speedsensor 30 is connected to the electronic stability control unit 14 viathe vehicle communication bus, which communicates with the electronicstability control unit 14 only periodically, for example every 20milliseconds. In contrast, the wheel speed sensor 22 is directly wiredto the electronic stability control unit 14, and thus provides a higherinformation refresh rate. This higher refresh rate at higher vehiclespeeds can, in some circumstances, be critical to providing safe vehicleoperation by the electronic stability control unit 14.

Although the illustrated embodiments include the powertrain electroniccontrol unit 12 that is separate from the electronic stability controlunit 14 and communicates with the electronic stability control unit 14via a communication bus, the vehicle control system 10 is not limited tothis configuration. For example, in some embodiments, the powertrainelectronic control unit 12 and the electronic stability control unit 14may be included within a centralized vehicle control unit that includesmost or all vehicle electronic control units. In such a centralizedvehicle control unit, the powertrain electronic control unit 12 and theelectronic stability control unit 14 may be implemented as individualsub-processors or control systems controlled by a higher level processoror control system, or alternatively may be fully integrated within asingle processor or control system.

Selective illustrative embodiments of the vehicle control system aredescribed above in some detail. It should be understood that onlystructures considered necessary for clarifying the vehicle controlsystem have been described herein. Other conventional structures, andthose of ancillary and auxiliary components of the vehicle controlsystem are assumed to be known and understood by those skilled in theart. Moreover, while a working example of the vehicle control systemhave been described above, the vehicle control system is not limited tothe working examples described above, but various design alterations maybe carried out without departing from the vehicle control system as setforth in the claims.

I claim:
 1. A vehicle control system comprising a powertrain electroniccontrol unit; an electronic stability control system including anelectronic stability control unit that is connected to the powertrainelectronic control unit via a vehicle communication bus, a wheel; awheel speed sensor that is configured to detect a speed of the wheel; anelectric motor that is configured to drive the wheel; and an electricmotor speed sensor that is configured to detect the speed of theelectric motor; wherein the vehicle control system is connected to acomputer readable storage medium having data stored therein representingsoftware executable by the vehicle control system, the softwareincluding program instructions comprising that the electronic stabilitycontrol unit detects an output signal of the electric motor speed sensorwhich corresponds to a detected wheel speed, program instructionscomprising that when the detected wheel speed is greater than apredetermined wheel speed, the vehicle control unit controls theelectronic stability control system based on an output of the wheelspeed sensor, and program instructions comprising that when the detectedwheel speed is less than or equal to a predetermined wheel speed, thevehicle control unit controls the electronic stability control systembased on an output of the electric motor speed sensor.
 2. The vehiclecontrol system of claim 1, wherein the wheel speed sensor iselectrically connected to the electronic stability control unit via acommunication line that is separate from the vehicle communication bus,and the electric motor speed sensor is electrically connected to thepowertrain electronic control unit via the vehicle communication bus. 3.The vehicle control system of claim 1, wherein the wheel speed sensorincludes a tone ring having a plurality of metal teeth and that rotatesconcurrently with the rotation of the wheel, and a magnetic sensor thatis configured to detect the teeth of the tone ring during rotation ofthe wheel.
 4. The vehicle control system of claim 1 wherein the electricmotor speed sensor includes a sensor that configured to detect both arotation of a rotor of the electric motor and a direction of rotation ofthe rotor of the electric motor.
 5. The vehicle control system of claim1 wherein the electric motor speed sensor is a Hall effect sensor. 6.The vehicle control system of claim 1 wherein the vehicle communicationbus is a controller area network vehicle bus.
 7. The vehicle controlsystem of claim 1, wherein the electronic stability control systemcomprises at least one of art antilock braking system including anantilock braking system control unit, a traction control systemincluding a traction control system control unit, and a vehicle dynamiccontrol system including a vehicle dynamic control system control unit.8. The vehicle control system of claim 1, wherein the electric motor iscontrolled by the powertrain electronic control unit via the vehiclecommunication bus.
 9. The vehicle control system of claim 1, comprisinga battery that supplies power to the electric motor.
 10. The vehiclecontrol system of claim 1, wherein the predetermined wheel speedcorresponds to a vehicle speed of 2 kilometers per hour.
 11. A method ofcontrolling a vehicle control system, the vehicle control systemcomprising a powertrain electronic control unit; an electronic stabilitycontrol system including an electronic stability control unit that isconnected to the powertrain electronic control unit via a vehiclecommunication bus; a wheel; a wheel speed sensor that is configured todetect a speed of the wheel; an electric motor that is configured todrive the Wheel; and an electric motor speed sensor that is configuredto detect the speed of the electric motor, the method comprisingdetecting an output signal of the electric motor speed sensor whichcorresponds to a detected wheel speed, and when the detected wheel speedis greater than a predetermined wheel speed, the electronic stabilitycontrol unit controls the electronic stability control system based onan output of the wheel speed sensor, and when the detected wheel speedis less than or equal to a predetermined wheel speed, the electronicstability control unit controls the electronic stability control systembased on an output of the electric motor speed sensor.
 12. A vehiclecontrol system comprising a vehicle control unit that provides andcontrols a powertrain electronic control system and an electronicstability control system; a wheel; a wheel speed sensor that isconfigured to detect a speed of the wheel; an electric motor that isconfigured to drive the wheel; and an electric motor speed sensor thatis configured to detect the speed of the electric motor, wherein thevehicle control unit is connected to a computer readable storage mediumhaving data stored therein representing, software executable by thevehicle control unit the software including program instructionscomprising that the vehicle control unit detects an output signal of theelectric motor speed sensor which corresponds to a detected wheel speedprogram instructions comprising that when the detected wheel speed isgreater than a predetermined wheel speed, the vehicle control unitcontrols the electronic stability control system based on an output ofthe wheel speed sensor, and program instructions comprising that whenthe detected wheel speed is less than or equal to a predetermined wheelspeed, the vehicle control unit controls the electronic stabilitycontrol system based on an output of the electric motor speed sensor.13. A vehicle control system comprising a powertrain electronic controlunit; an electronic stability control system including an electronicstability control unit that is connected to the powertrain electroniccontrol unit via a vehicle communication bus, a wheel; a wheel speedsensor that is configured to detect a speed of the wheel; an electricmotor that is configured to drive the wheel; and an electric motor speedsensor that is configured to detect the speed of the electric motor;wherein the vehicle control system is connected to a computer readablestorage medium having data stored therein representing softwareexecutable by the vehicle control system, the software including programinstructions comprising that the electronic stability control unitdetermines an output signal of the electric motor speed sensor whichcorresponds to a wheel speed, program instructions comprising that whenthe output signal corresponding to a wheel speed is greater than apredetermined wheel speed, the electronic stability control unitcontrols the electronic stability control system based on an output ofthe wheel speed sensor, and program instructions comprising that whenthe output signal corresponding to a wheel speed is less than or equalto a predetermined wheel speed, the electronic stability control unitcontrols the electronic stability control system based on an output ofthe electric motor speed sensor, Wherein the Wheel speed sensor includesa tone ring having a plurality of metal teeth and that rotatesconcurrently with the rotation of the wheel, and a magnetic sensor thatis configured to detect the teeth of e tone ring during rotation of thewheel, and wherein the electronic stability control system comprises atleast one of an antilock braking system including an antilock brakingsystem control unit, a traction control system including a tractioncontrol system control unit, and a vehicle dynamic control systemincluding a vehicle dynamic control system control unit.